Biological thiols perform significant functions. For example, glutathione (GSH) serves as an antioxidant. It protects cells from oxidative stress by trapping free radicals that damage DNA and RNA. It also has important roles in maintaining the reducing environment in cells, catalysis, metabolism, transport, and gene regulation. GSH is the most abundant of the low molecular weight cellular thiols. It is present in millimolar concentrations; however, extracellular concentrations are relatively low, e.g., 2-20 μM in plasma. GSH is related to several diseases thought to be mediated by oxidative stress, including Alzheimer's disease, acquired immune deficiency syndrome, psoriasis, cancer, cardiovascular disease, heart attack, liver damage, leukocyte loss, Parkinson's disease, and stroke. Cysteine (Cys) is involved in protein synthesis, detoxification, and metabolic processes. Abnormal levels of Cys are related to many disorders, such as impaired growth, Alzheimer's disease, and cardiovascular disease. Cys deficiency is involved in slowed growth, hair depigmentation, edema, lethargy, liver damage, muscle and fat loss, skin lesions, and weakness (Shahrokhian, Anal. Chem. 2001, 73:5972-5978).